1. Field of the Invention
The present invention relates to a power module substrate, which is employed in a semiconductor device controlling a large amount of high voltage electrical current, a power module including the power module substrate, and a method for manufacturing the power module substrate.
This application is based on and claims priority from Japanese Patent Application No. 2008-149902, filed on Jun. 6, 2008, Japanese Patent Application No. 2009-065033, filed on Mar. 17, 2009, Japanese Patent Application No. 2009-075315, filed on Mar. 26, 2009, Japanese Patent Application No. 2009-086247, filed on Mar. 31, 2009, and Japanese Patent Application No. 2009-086248, filed on Mar. 31, 2009, the contents of which are incorporated herein by reference.
2. Background Art
Conventionally, in all of semiconductor elements, a power module is used for the power supply.
The amount of heat generated by the power module is relatively high.
Consequently, as a substrate on which the power module is mounted, a power module substrate is used in which a metal plate made of Al (aluminum) is joined to a ceramics substrate composed of, for example, AlN (aluminum nitride), Si3N4 (silicon nitride), or Al2O3 (aluminum oxide), with an Al—Si system brazing filler metal interposed therebetween.
In addition, the metal plate is formed as a circuit layer, and a semiconductor chip that is a power element is mounted on the metal plate with a solder material interposed therebetween.
In addition, in order to improve the heat radiation efficiency, a structure in which a metal layer is formed by connecting a metal plate composed of Al or the like with a lower face of a ceramics substrate, and the entire power module substrate is joined to a heat radiation plate with the metal layer interposed therebetween has been proposed.
Conventionally, in order to reliably obtain the joint strength between metal plates which serve as the circuit layer and the metal layer, and a ceramics substrate, for example, technique of having the surface roughness of the ceramics substrate being less than 0.5 μm has been known, as disclosed in Japanese Unexamined Patent Application, First Publication No. H3-234045.
However, when the metal plate is joined to the ceramics substrate, even if the roughness surface of the ceramics substrate is simply reduced, sufficient high joint strength is not obtained and there is a disadvantage in that the reliability thereof cannot be improved.
Even if, for example, a honing treatment is performed on the surface of the ceramics substrate by use of Al2O3 particles in a dry method and the roughness surface Ra thereof is made 0.2 μm, peeling may occur at an interface thereof in a peeling test.
In addition, even if a ceramics substrate is polished by use of a polishing method so that the roughness surface Ra is made less than or equal to 0.1 μm, there is a case where peeling occurs at the interface in the same manner as described above.
In addition, in a case where a power module substrate is subjected to a heat-load cycle, not only peeling at an interface but also cracks being generated in the ceramics substrate is known.
Specifically, recently, in power modules, downsizing and reducing of thickness has been required, and the usage environment has become severe.
The power module is used under a usage environment in which, for example, heat stress is repeatedly generated.
In addition, recently, the amount of heat generated in an electronic component has tended to increase, so it is necessary to dispose a power module substrate on a heat radiation plate as described above.
In this case, since the power module substrate is rigidly fixed to the heat radiation plate, a large shear force is generated at a joint interface between the metal plate and the ceramics substrate when the substrate is subjected to a heat-load cycle.
As a result, improvement of the joint strength and reliability are further required.